In a message dated 12/6/04 20:35:55, [log in to unmask] writes:
> Techies,
> Au-content < =3%. If you are above, AuSn4 intermetalics will form and make
> the solder joint brittle.
> In general there are the following intermetalics: AuSn, AuSn2, AuSn4, Ag3Sn,
> Cu3Sn, Cu6Sn5, Ni3Sn2, Ni3Sn4, Ni3Sn7 (it is soldering, therefore all
> contain tin). AuSnx and AgySn are "weak". AuSn can form due to "Ausscheidungen" at
> the "Korngrenzen" (sorry, perhaps somebody can translate) at room temperature
> and below the 3% limit. This will also weaken the solder joint, therefore be
> careful for applications with high reliability requirements. Perhaps Mr.
> Reliability (Werner Engelmaier) could comment on this and translate the german
> words.
> Dissolution from gold in tin is real quick, at least 1000 times quicker than
> Ni or app. 30 times faster than Cu dissolves in tin. Therefore the time the
> solder stays heated shouldn´t be an issue, at least in a reflow process. For
> wave soldering process the gold from pcb finish will "disappear" in the
> solder pot.
> Gebhard
>
Natürlich, Gebhard,
"Ausscheidung" is precipitation, and "Korngrenzen" are grain boundaries.
> Actually, AuSn, AuSn2, AuSn4 will form at any amount of Au-content;
> however, the concentration of Au/Sn-IMCs is the critical issue, and ~3wt% is about
> right--no real science behind this.
>
> Regards,
> Werner Engelmaier
> Engelmaier Associates, L.C.
> Electronic Packaging, Interconnection and Reliability Consulting
> 7 Jasmine Run
> Ormond Beach, FL 32174 USA
> Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
> E-mail: [log in to unmask], Website: www.engelmaier.com
>
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